BOSTON — Moises Kaufman's "33 Variations" is a big play about big ideas. And the biggest idea is this: Don't dismiss the small stuff.

Any play with Beethoven at its core is bound to be big. As big as his music, as big as his personality, as big as the pathos of his ultimate deafness. But Kaufman, unafraid of big ideas as evidenced by his previous plays, "The Laramie Project" and "Gross Indecency: The Three Trials of Oscar Wilde," uses his protagonist to explore something seemingly smaller about the composer.

Musicologist Katherine Brandt (Paula Plum, absolutely masterful) wants to know why Beethoven was so intrigued by the apparently nondescript little waltz written by Viennese music publisher Anton Diabelli that he wrote, over the period of several years, 33 variations based upon it. Despite the fact that she is entering the last stages of ALS, or Lou Gehrig's disease, she travels to Bonn, Germany, to research the composer's original notes.

Why was a genius like Beethoven so obsessed with what Brandt considers a "cobbler's patch" of a work. "What was it about this mediocre waltz that captivated his imagination?" she asks.

"Mediocre" is a word Brandt uses often, particularly in reference to her twentysomething daughter, Clara, played with wistful yearning by Dakota Shepard, a veteran of Wellfleet Harbor Actors Theater and Cape Rep stages. Katherine is the type of mother who tells her daughter she has beautiful hair, then slips in the "I wish you'd do something with it" zinger. Clara is the type of daughter who is stubbornly devoted to such a mother.

That begins to change when Clara takes Katherine for a checkup and they meet Mike (a sweetly winning Kelby T. Akin), a male nurse who helps Katherine with her physical therapy and Clara with her self-esteem. After some charmingly awkward exchanges, the young couple fall in love. Of course, Katherine sees Mike as one more mediocre personality, hardly the type of man who is going to elevate Clara. She is so wrong.

Against this backdrop of modern life Kaufman interposes a trio of 19th century personalities: Beethoven (James Andreassi, who brings the house down in Act 2 when he dramatically takes us through the composition process); Diabelli (the talented Will McGarrahan, last seen in Lyric's "The Chosen" and "The Temperamentals"); and Beethoven's harried but infinitely patient assistant Anton Schindler (a charismatic Victor L. Shopov). Diabelli has written a waltz and invited other composers to submit variations, which his firm will compile and publish. Many comply. But Beethoven, who initially rejects the idea, becomes obsessed with the little piece, ultimately producing 33 remarkable variations. As years pass, Beethoven cannot let go of the waltz — there is always another variation hiding within its notes — and Diabelli becomes increasingly frantic he will never see a final product.

Meanwhile, in Bonn, Katherine has become friendly with Beethoven library archivist Gertrude Ladenburger (Maureen Keiller, providing a perfect deadpan foil for Katherine). The scenes of the two women lovingly turning the pages of the master's notebooks, as they are projected onto screens for the audience and as the works themselves are played by pianist Catherine Stornetta, border on magical, even for those who can't read a note of music. But Katherine gets no closer to solving the mystery of the "Variations." Did Beethoven just write them for the money? Or to one-up Bach, who had once written 32 variations on a piece? As Katherine becomes increasingly frustrated by her now rapid physical deterioration (a process Plum delivers with poignant perfection), she is equally confounded by her inability to see what mesmerized the maestro.

Perhaps, Gertrude suggests in a perhaps too tidily sewn up solution, the mystery is not about Beethoven but about Katherine's judgmental nature, in which "everything you don't understand is mediocre." And that includes Clara, she says, a daughter she believes Katherine regards as "a second-rate waltz."

It is only when Katherine looks deeper — the way Beethoven looks at the second-rate waltz — that she can appreciate, before it is too late, the small beauties and everyday wonders that life offers.

Beethoven's "Variations," she ultimately writes in a lecture she asks Clara to deliver, "show us what lies in every moment of the waltz." They allow us to "enter the minutiae that life, in its haste, robs us of."